An array of image sensors or light sensitive sensors detect the intensity of light received by the image sensors. The image sensors typically generate electronic signals that have amplitudes that are proportionate to the intensity of the light received by the image sensors. The image sensors can convert an optical image into a set of electronic signals. The electronic signals may represent intensities of colors of light received by the images. The electronic signals can be conditioned and sampled to allow image processing.
Integration of the image sensors with signal processing circuitry is becoming more important because integration enables miniaturization and simplification of imaging systems. Integration of image sensors along with analog and digital signal processing circuitry allows electronic imaging systems to be low cost, compact and require low power consumption.
Historically, image sensors have predominantly been charged coupled devices (CCDs). CCDs are relatively small and can provide a high-fill factor. However, CCDs are very difficult to integrate with digital and analog circuitry. Further, CCDs dissipate large amounts of power and suffer from image smearing problems.
An alternative to CCD sensors are active pixel sensors. Active pixel sensors can be fabricated using standard CMOS processes. Therefore, active pixel sensors can easily be integrated with digital and analog signal processing circuitry. Further, CMOS circuits dissipate small amounts of power.
FIG. 1 shows a cross-section of a prior art array of image sensors. This array of image sensors includes PIN diode sensors located over a substrate 10. An interconnection structure 12 electrically connects an N-layer 14 of the PIN diodes to the substrate 10. An I-layer 16 is formed over the N-layer 14. A P-layer 18 is formed over the I-layer 16. The P-layer 18, the I-layer 16 and the N-layer 14 form the array of PIN diode sensors. A first conductive via 20 electrically connects a first diode sensor to the substrate 10, and a second conductive via 22 electrically connects a second diode sensor to the substrate 10. A transparent conductive layer 24 is located over the array of diode sensors. A conductive lead 26 is connected to the transparent conductive layer 24. The conductive lead 26 is connected to a bias voltage which allows biasing of the P-layer 18 of the array of PIN diode sensors to a selected voltage potential.
A limitation of the image sensor structure of FIG. 1 is the electrical connection between the conductive lead 26 and the transparent conductive layer 24. The transparent conductive layer 24 must be electrically conductive to allow biasing of the PIN diodes, and must be transparent to allow the PIN diodes to receive light. Generally, it is very difficult to bond to the types of materials that must be used to form the transparent conductive layer 24. Therefore, the conductive lead 26 must be attached to the transparent conductive layer 24 with the aid of some type of clamp or support structure. The result being an electrical connection which is not reliable and which is expensive to produce.
Another limitation of the image sensor structure of FIG. 1 is that the individual image sensors are not isolated from each other. That is, light received by a given image sensor will effect neighboring image sensors because current can flow through the N-layer 14 between neighboring image sensors. Charge can flow between the image sensors especially when the light intensity of the received light varies greatly between neighboring image sensors. The P-layer 18, the I-layer 16 and the N-layer 14 are shared by neighboring image sensors. A trench 28 is formed to provide some isolation between the image sensors by increasing the resistance between the N-layers sections of neighboring image sensors.
It is desirable to have a plurality of active pixel sensors formed adjacent to a substrate in which a transparent conductive layer is reliably electrically connected between the pixel sensors and to a pixel sensor bias voltage which originates on the substrate. It is also desirable that the pixel sensors be isolated from each other to reduce coupling between the pixel sensors.